Trenches formed in the substrate and filled with oxide are widely used as isolation structures. Shallow trench structures can be used to isolate adjacent electronic devices, such as transistors. Deep trench isolation structures can be used to isolate N-wells and P-wells in the substrate. The trench isolation technology has proved to be important in fabricating large scale integrated circuits that require high density of electronic devices.
Trench isolation structures may be formed in a semiconductor substrate by depositing silicon dioxide in the isolation trench. A silicon dioxide layer is thermally grown over the surface of a silicon substrate. Next, a silicon nitride layer is deposited by chemical vapor deposition. The layers are then patterned to define the width of a trench to be etched in the silicon substrate. The exposed surface, in this case silicon nitride, is then removed using plasma or chemical etching. Similarly, silicon dioxide is removed by chemical or plasma etching. Following this, the exposed surface of the substrate, in this case silicon, is further removed to create a trench.
After a desired trench depth is obtained, the etch process stops. A thin silicon oxide layer is thermally grown in the trench area. This trench oxide layer is formed to passivate the interior walls and bottom of the trench which may have been slightly damaged during the trench etch.
Upon completion of the trench oxide layer, deposited silicon dioxide is used to fill the trench. After the deposition of silicon dioxide, the substrate is planarized. During planarization, silicon dioxide is removed completely over the silicon nitride hard mask. The silicon nitride layer is subsequently etched away. In this structure, deposited silicon dioxide covers the trench area, whereas thermally grown silicon dioxide, called pad oxide, covers the non-trench area.
In a subsequent step of etching away the pad oxide layer, grooves and notches indicated as "A" may be created in the trench oxide surface area 60a, which is illustrated in FIG. 2.
A sacrificial oxide layer may be thermally grown onto a substrate after trench formation. After implantation, the sacrificial oxide layer typically is removed by chemical etching.
Because thermally grown silicon dioxide can only form in the area where there is silicon, this grown silicon dioxide layer covers only the non-trench area. As a result, the trench area is not covered by additional silicon dioxide. When the sacrificial oxide layer 80a is subsequently etched away, some of the trench fill oxide 60a will be etched away, as indicated at "A" in FIG. 3.
The formation of grooves and notches of this sort may adversely impact the operability and reliability of an integrated circuit.